In petroleum processing, aromatic streams are derived from processes such as naphtha reforming and thermal cracking (pyrolysis). These aromatic streams also contain undesirable hydrocarbon contaminants including mono-olefins, dienes, styrenes and heavy aromatic compounds such as anthracenes.
The aromatic streams are used as feedstocks in various subsequent petrochemical processes. In certain of these processes, such as para-xylene production, e.g., from an aromatic stream containing benzene, toluene and xylenes (BTX) or toluene disproportionation, hydrocarbon contaminants cause undesirable side reactions. Therefore the hydrocarbon contaminants must be removed before subsequent processing of the aromatic streams.
Moreover, the shift from high-pressure semiregenerative reformers to low-pressure moving bed reformers results in a substantial increase in contaminants in the reformate derived streams. This in turn results in a greater need for more efficient and less expensive methods for removal of hydrocarbon contaminants from the aromatic streams.
Undesirable hydrocarbon contaminants containing olefinic bonds are quantified by the Bromine Index (BI). Undesirable olefins, including both dienes and mono-olefins, have typically been concurrently removed from aromatic streams such as BTX by contacting the aromatic stream with acid-treated clay. Other materials, e.g., zeolites, have also been used for this purpose. Clay is an amorphous naturally-occurring material, while zeolites used for this purpose generally are synthesized and are therefore more expensive. Both clay and zeolites have limited lifetimes in aromatics treatment services. Although clay is much cheaper per se than zeolites, under certain circumstances the choice of zeolites can provide improved performance, making the selection of zeolites an overall better economic choice.
Improvement in using zeolites to remove contaminants from aromatic feeds have been described in numerous patents, such as U.S. Pat. Nos. 6,368,496; 6,500,996; 6,781,023; 7,214,840; 7,517,824; 7,731,839; 7,744,750; U.S. Patent Application Publications 2007-0112239 and 2008-0128329; and U.S. Provisional Application 61/240,424.
Olefin removal devices in use today, designed for use with clay, do not use a feed distribution system. This is because feed distribution does not improve cycle length, as clay beds age in a band-wise fashion. Accordingly, a feed distribution system would just be an additional cost without any benefit. Furthermore, clay can be completely exhausted at a reaction temperature of 205° C. Reactor design heretofore typically provides for a maximum temperature of about 210° C. Increasing the reactor temperature enables only small gains in run length. Achieving a higher temperature requires use of hotter steam in the feed heater and addition of a feed effluent heat exchanger since the product is too hot for optimal integration with aromatics distillation towers.
The present inventor has discovered a process and apparatus for the practice thereof, whereby trace olefins and dienes are removed from aromatic plant feedstocks using a reactor design that enables the product to be backmixed with the feedstock and that has a feed/effluent heat exchanger.